This invention relates to closed loop compensated frequency references and, in particular, to a closed loop compensated frequency reference containing a minimum mean error frequency source.
There are many current applications for oscillator circuits that require a high degree of accuracy in their output. An example of an application is in a secure communications system which requires a time-of-day clock.
In the prior art, the stringent accuracy requirements for an oscillator such as a crystal oscillator or a voltage-controlled oscillator require the selection of precision components, elaborate temperature control schemes, and relatively large quantities of power to drive the circuits. In the case of a portable device, such as a portable radio, that is required to synchronize its transmission with a precision time-of-day clock, the aforementioned requirements for building a precision clock oscillator become very prohibitive, not only in the area of expense, but also in the area of power consumption. The accuracy for a time-of-day clock that is used to synchronize a field radio that periodically changes frequencies, often is beyond the capabilities of conventional temperature compensated crystal oscillators. Thus, it is very difficult to provide an accurate time-of-day clock that is reasonably priced and operates on relatively low power.
Additionally, temperature compensation of such an oscillator driven clock, especially a crystal oscillator, normally involves measuring the temperature of the crystal, determining the frequency error of the crystal at the temperature and then applying a correction to a control signal which pulls the oscillator back on frequency. This, of course, results in many errors and because of the type of crystal which is normally used in this type of circuit, there are accelerated aging rates of the crystal which must also be compensated for, as well as frequency hysteresis effects.
U.S. Pat. No. 4,305,041 discloses a time compensated clock oscillator that has a low power oscillator that drives the time-of-day clock and a second accurate oscillator that is used to measure the frequency of the first oscillator. A correction signal is developed from the mixing of the two oscillators to periodically adjust the time-of day clock that is being driven by the circuitry.
Temperature compensation of a closed loop compensation reference oscillator was provided in my U.S. Pat. No. 4,297,657 which disclosed an oscillator that is temperature compensated by adjusting a slave oscillator with an error signal that is produced by using the temperature of a reference oscillator to determine the frequency error of the reference oscillator and then adjusts the frequency of the slave oscillator until its frequency coincides with the desired nominal value. The first of the above referenced oscillators required the provision of a data interface for interactions between the frequency source and the time-of-day clock while the latter requires that the measurement circuitry operate with a short duty cycle.